It is known that straight-chain monoolefin may be separated from branched-chain monoolefin by processes employing molecular sieve having a pore diameter of about 5 Angstroms. These processes are based on the well known acceptance-exclusion principle based on molecular size. A molecular sieve which has a pore diameter of about 5 Angstroms, will accept or adsorb straight-chain monoolefin. Such processes are described, for example, in U.S. Pat. No. 3,721,064 to Symoniak et al. and U.S. Pat. No. 3,717,572 to de Gramont et al.
Such processes may be used for the separation of straight-chain monoolefin from branched-chain monoolefin from feed mixtures of the four, five, and six carbon monoolefins. Such processes are particularly useful for the separation of mixed butene feeds into straight-chain butene monoolefin and branched-chain isobutylene monoolefin. Isobutylene is used, for example, in the manufacture of butyl rubber, polyisobutylenes and in the production of gasoline alkylate. Straight-chain normal butene monoolefins include butene-1, trans-butene-2, and cis-butene-2 which are used, for example, in the production of secondary butyl alcohol and methyl ethyl ketone. In some instances, the straight-chain butenes product is further separated, e.g., by fractional distillation, to obtain a butene-1 product which may be used, for example, as in the manufacture of polyethylene copolymers.
If a monoolefin feed contains butenes, pentenes and hexenes, it is considered advantageous to prefractionate the feed and operate the separation process on a monoolefin feed having a single carbon number.
A suitable molecular sieve for use in such a separation process as an adsorbent is synthetic zeolite type A in its calcium cation exchanged form known as type 5A or its high calcium cation form. Type 5A and high calcium exchanged type 5A molecular sieve may be obtained from Union Carbide Corporation, New York, N.Y. Other useful zeolites of natural origin or synthesized having pore sizes of about 5 Angstroms include chabazite, mordenite, gmelinite, erionite and those known as types D, R, S, and T.
In general, the separation process comprises an adsorption step wherein a hydrocarbon vapor feed stream containing straight-chain monoolefin and branched-chain monoolefin is passed into one end of an adsorber containing a molecular sieve having a pore diameter of about 5 Angstroms. The molecular sieve has adsorbed thereon straight-chain paraffinic hydrocarbon, most suitably normal hexane. A first effluent stream is obtained from the other end of the adsorber. The first effluent stream contains branched-chain monoolefin and the straight-chain paraffin hydrocarbon. Straight-chain monoolefin has been adsorbed by the molecular sieve.
A copurge or cocurrent purge step is then practiced. A straight-chain paraffinic hydrocarbon vapor stream, most suitably normal hexane, is passed into said one end of the adsorber and a second effluent stream comprising branched-chain monoolefin and straight-chain paraffin hydrocarbon is obtained from said other end of the adsorber. Straight-chain paraffin hydrocarbon is adsorbed by the molecular sieve. Most suitably, sufficient molecular sieve adsorbent has not been utilized in the adsorption step so that substantially all the straight-chain monoolefin will remain adsorbed in the adsorbent bed and will not break through and exit with the second effluent stream. The copurge step is suitably continued until substantially all of the branched-chain monoolefin has exited the adsorber in the second effluent stream.
Next, a desorption or countercurrent purge step is practiced. A straight-chain paraffinic hydrocarbon vapor stream, most suitably normal hexane, is passed into said other end of the adsorber. A third effluent stream comprising straight-chain monoolefin and straight-chain paraffinic hydrocarbon is obtained from said one end of the adsorber. The countercurrent purge step is suitably continued until substantially all of the straight-chain monoolefin has exited the absorber in the third effluent stream. The straight-chain paraffinic hydrocarbon is adsorbed on the molecular sieve during the countercurrent purge step.
The absorption step is again practiced and the process cycle repeated as desired.
The branched-chain monoolefin and straight-chain paraffinic hydrocarbon in the first and second effluent streams are separated, e.g., by fractional distillation, to obtain as overhead a purified branched-chain monoolefin product and as bottoms a straight-chain paraffinic hydrocarbon.
The straight-chain monoolefin and straight-chain paraffinic hydrocarbon in the third effluent stream are separated, e.g., by fractional distillation, to obtain as overhead a purified straight-chain monoolefin product and as bottoms a straight-chain paraffinic hydrocarbon.
It is very desirable, from an economic and process efficiency viewpoint, to recycle the separated straight-chain paraffinic hydrocarbons as purge for use in subsequent process cycles.
It is known that such a separation process results in the formation of polymers. Polymer which forms in the adsorber appears in the separated straight-chain paraffinic hydrocarbon which is going to be recycled for purge. As a result, there will be a continuous buildup of polymer in the recycled straight-chain paraffinic hydrocarbon purge medium unless polymer is removed. Such a polymer build up is disadvantageous because it can lead to adsorbent deactivation.